Alt voltage low...

I have twin batteries on my 90 with an alternator each. The main alt is a disco 300 tdi 100amp alt and the second is a disco 200tdi 65amp.

A few weeks ago both of these were kicking out mid to high 13v and I had plenty of power for winching however they are both struggling around 12.1v now, could this be a poor earth between the alt and block or block itself? I just find it bizarre how they are both pretty poor in comparison to what they were kicking out a few weeks ago. I have a twin motor 8274 on the front of my 90 which pulls alot of draw when it wants to get going and without both alts churning away nicely it makes the winch feel pretty lifeless

If you are only getting 12.1v then I would say they aren't charging at all and what you are seeing is the actual battery voltage. Do you have battery warning lights for both alternators, and if so do they work as expected (lit up with ignition on and out with engine running?) Also worth measuring battery voltage with it stopped and running to see if there is any difference. You could always try the old jump lead test between the block and battery -ve terminal.

That doesn't sound too healthy. The fact that they've apparently both gone at once suggests it might be in the connections rather than the alternators themselves, unless it is a spooky coincidence. Is your 12.1v showing up at the battery terminals? Or at the alternators' positive terminals? I had an apparent loss of charge once when my alternator positive terminal had come loose. It didn't show up on the dash, because the that was fed by a different terminal.

U have dual volt gauges. I've not long put them on for the same reason that the old ones were past their sell by date. I changed the regulator pack in the 65 and used a known working alt from a disco. They both have separate exciter lights ect, they were kicking some out as with the led light bar ect on it was still about 12.1 but before I could run the light bar n whatever else I needed to and still have 13v. I added another earth to the aux battery maybe the main could do with one or two more or even the alts themselves arnt making a good connection... I'll have a play within the next couple of weeks and see what I find

it should be charging at 14.4v at 13 it wont charge.

Yes, with the engine running see what a meter shows between the big positive terminals on the alternators and the engine itself. If that's in excess of 14 v then the alternator is alright. Then try positive terminal to chassis, positive terminal to battery negative etc to narrow down the source of the problem. Of course you may not see 14.5 v anywhere, in which case it's an alternator problem. The fact that thy both seem to have gone down to 12.1 v at once implies that there's a poor connection between the engine and the chassis or between engine and battery negative. A fully charged battery in good condition can show you more than 12.1 v, so it sounds like your batteries are running down too!

Also worth sticking your batteries on a charger as a temporary measure to save them sulphating up and loosing capacity if it is going to be a while before you can fix it - also saves you breaking down from a flat battery if they aren't charging.

Both dropping to the same voltage is pretty strange, the winch isn't trying to operate or something stupid like that is it and it's just voltage drop due to the high current load.

Is your meter maybe duff?

two separate volatage meters, two separate alts, separate earths from battery negatives to chassis, winches are all isolated and turned off. common thing between them would be the engine earth. Ill drop the charger on it tonight and also do the checks that @Brown has suggested then report back thanks

OK quick update on this as I havnt done anything on the truck for a while...

Tonight I added an earth from the second alt body to my tube front wings which are welded to the chassis. This made the alts charge to 13.2v on tick over and then I added a large earth from the main battery to the aux battery, as the aux battery has two earth's from it, still 13.2v on both however the second alt went to 13.7with a little revs.

Tomorrow I'll add an earth to the main alt body to chassis and also clean my chassis to main battery strap....
I don't think I've ever seen 14v on any of my alts or on this truck ever thinking about it. My other 90 charges lovely at 14v always